1. Field of the Invention
This invention relates to a method and apparatus for detecting a pilot signal in a mobile communication system of the code division multiple access system, in which spread-spectrum signals transmitted from respective base stations are received and pilot signals are detected from the received spread-spectrum signals. The invention also relates to a subscriber for modulating the received spread-spectrum signals based on the pilot signal detected by the pilot signal detection method and apparatus.
2. Description of the Related Art
Among the communication systems used when a mobile station is used in common by plural base stations for communication, there is a code division multiple access (CDMA) system. This CDMA system is a system in which a signal modulated by a carrier frequency is spectrum-spread in accordance with a pre-set PN code and transmitted to, for example, a mobile station, and in which a receiver demodulates the signal with code synchronization using the same PN code for communication. In the mobile communication system employing the CDMA system, a subscriber (receiver of a mobile station) allocates the timing of the pilot signal transmitted by the base station to a demodulator which then demodulates the spectrum spread signal transmitted from the base station based on the timing of the allocated pilot signal.
The mobile communication system employing the CDMA system has already been standardized in the United States. The system standard in the subscriber prescribes the overall performance of the mobile station, including demodulation properties, such as message error rate (MER) as a result of demodulation at the subscriber in case the signal intensity of the pilot signal from plural base stations is varied in accordance with a specified pattern, or reliability of the message on the communication protocol to be transmitted by the mobile station as a result of recognition of a neighbor base station. However, there is no provision as to the method for detecting the timing of the pilot signal from the neighbor base station.
The subscriber performance can possibly be improved if, when detecting the timing of the pilot signals of the neighbor base stations, correlation of the pilot signal transmitted by the neighbor base station is taken perpetually and, on detection of a pilot signal from the neighbor base station not lower than a pre-set level, a message prescribed on the communication protocol is transmitted, at the same time as the timing of the detected pilot signal is immediately allocated to the demodulator.
However, if, at the time of detection of the timing of the pilot signal by the subscriber, the range of partial correlation is set to a larger value, the time required for partial correlation becomes larger in proportion to the size of the range of the partial correlation, although the detection accuracy of the timing of the pilot signal is improved. Moreover, the subscriber has to execute processing for correlation of the currently received pilot signal of the base station, allocation of the pilot signal to the timing demodulator, processing of the communication protocol and speech encoding/decoding, in addition to the above-described timing detection of the plot signal. Therefore, if it is attempted to improve accuracy in timing detection of the pilot signal, the processing time therefor is increased, a result of which the overall performance of the subscribed cannot be improved.
Thus it has been desired to develop an accurate and efficient method for detecting the timing of the pilot signal of the neighbor base station at the mobile station.